Traffic Alert and Collision Avoidance Systems (TCAS) have been operational in aircraft for a number of years. TCAS may determine, on a first aircraft, whether an approaching aircraft will come within a minimum distance of a position of the first aircraft. TCAS equipment on the first aircraft may interrogate a transponder on an in-range second aircraft and listen for the transponder reply. By computer processor analysis of these replies, the airborne TCAS processor may display all TCAS targets within a certain range and may determine if one of the TCAS targets may represent a potential collision threat. If so, TCAS may provide indications (both visual and aural) to a pilot to offer separation guidance. TCAS may use radio frequency (RF) transmissions between the two aircraft to determine range and bearing. The altitude of the other aircraft is obtained through the interrogation response. Using subsequent interrogations, TCAS determines the closure rate and the altitude rate of the other aircraft.
The RF signals are transmitted via two antennas: one antenna mounted on an external upper surface of the aircraft and one mounted on an underside of an aircraft. Traditionally, the antennas used by TCAS include a directional antenna mounted on the top of the aircraft and an omnidirectional antenna mounted on the bottom of the aircraft. Two antennas may be required for TCAS certification from various traffic control entities in whose airspace the aircraft may fly.
The directional antenna may comprise a four-beam antenna capable of individual power outputs in each directional beam. Traditionally, each beam may transmit in a 90 degree horizontal azimuth pattern with a vertical elevation of approximately plus and minus 20 degrees. Each beam may transmit at an individual power level, with the forward beam the strongest, to ensure necessary coverage. The RF signals received by the directional antenna may allow the TCAS processor to determine the relative bearing to the other aircraft.
An omnidirectional antenna may also transmit and receive RF energy providing input to the TCAS processor. Traditionally, the omnidirectional antenna has been mounted on the underside of the aircraft. An omnidirectional antenna may provide the same level of safety for collision avoidance, but does not provide bearing information. Hence, any interrogation responses only received by the omnidirectional antenna will not provide bearing information. This configuration may have proven beneficial in the past, where the top of the aircraft does not have as many obstructions, such as landing gear. Thus, the directional antenna mounted on the aircraft top may receive the more critical signal.
RF energy may be inhibited by a metal physical structure. One of the reasons for a traditional dual antenna configuration may stem from the fact that the aircraft structure may block RF energy. RF signals transmitted from an antenna mounted on the bottom of an aircraft may be partially or completely blocked by the aircraft structure keeping the signals from reaching an aircraft at a higher altitude than the transmitting aircraft. Similarly, signals transmitted from a higher aircraft may not reach an antenna mounted to the bottom of a lower altitude aircraft. For this reason, both top and bottom antennas are required for TCAS processing. The TCAS processor uses both antennas to maintain a complete picture of the traffic around the aircraft to which it is mounted.
On a typical fixed wing aircraft, there are more physical obstructions on the bottom of the aircraft than there are on the top. The landing gear of a fixed wing aircraft is typically the greatest obstruction to a clear view around the aircraft. As such, the directional antenna has historically been mounted on the top of an aircraft as in that location, there are fewer obstructions.
On certain aerial vehicles, there are numerous operationally required but RF blocking fixtures mounted on the top of the aircraft. For example, the rotor on a helicopter may be constructed of material not suitable for transmission of RF energy. Although signals received from an aft direction may be not as critical, a helicopter tail rotor may extend vertically to inhibit RF energy in the aft direction. On other aircraft types for example, vertically extending structures such as a vertical tail or a high mounted elevator on the tail may extend vertically such to inhibit TCAS reception from the aft direction. Also, other antenna elements and associated support structure may extend vertically above an aerial vehicle structure such to inhibit transmitted RF energy from a top mounted antenna.
Directional antenna performance may be further inhibited if the directional antenna may be mounted on a non-level surface. The elevation of transmission and reception of the directional antenna may be dependent upon the physical angle at which the antenna is mounted. A fixed antenna may transmit and receive at a fixed elevation. Therefore, if an antenna is mounted at an angle other than that angle at which the aircraft is traveling, the azimuth of transmission and reception of the antenna will be altered by the number of degrees of difference between the angle and the direction of aircraft travel. This alteration may adversely affect TCAS performance.
There may be few level surfaces on the upper fuselage of a particular aerial vehicle on which to mount a directional antenna. Top mounted engine cowlings, fuselage structure, and gearbox covers may require ease of maintenance access such that an antenna mounted to the covers may be impractical.
Directional antenna performance may also be degraded without a sufficient ground plane in proximity to the mounted antenna. A sufficient ground plane for accurate reception may typically be a 2 ft. radius circle around the mounted location of the antenna. The structure and aerodynamic design of some aerial vehicles may require no flat ground plane surface on the top of the aerial vehicle.
Therefore, a novel approach may be necessary to allow for unrestricted reception of a TCAS signal on an aircraft with uneven or undesirable RF blocking surface structure on the aircraft top. This novel approach may comprise locating the TCAS primary directional antenna on a position other than the top of an aircraft.